American University of Nigeria
Petroleum System and its Elements
October 2, 2014
In order to define petroleum system we have to define the two terms each alone and then combine both to get a real correct meaning of petroleum system. The word petroleum comes from the Latin petra, meaning “rock,” and oleum, meaning “oil.” It is also known as crude oil; therefore, Petroleum is a term used to represent mixture of gaseous, liquid, and solid hydrocarbon that occurs naturally beneath earth’s surface. Petroleum is used as a raw material of various products including natural gas, gasoline, naphtha, kerosene, fuel and lubricating oils, paraffin wax, and asphalt. Now we get the definition of petroleum let’s define system1. A system is a set of elements or components all working together to accomplish the same goal. Therefore, Petroleum System can be defined as a collection of geologic components and processes that play role in generation and accumulation of hydrocarbons.
Image showing a typical petroleum system2
Elements of Petroleum System
As illustrated on the previous figure Petroleum system consists of different elements. Those elements are: mature source rock, migration pathway, reservoir rock, trap and seal3.
1. Mature source rock
This is a rock from which petroleum can be generated or was generated in the past. Source rock is rich in organic matters which under adequate temperature decompose and produce hydrocarbons. Those organic matters are divided into three general categories, one is algae, a second is marine planktonic organisms and bacteria and the third is terrestrial plants (like leaves and stems). Source rocks are mainly shale; however, they can also be found as limestone, coal and other rocks that, under special circumstances, will occasionally generate hydrocarbons as well. Source rock can occur in different environment like deep water marine (ocean), lakes, deltas and swamps4. Source rock is one of indicators that tell people during exploration that petroleum may be available in that place.
2. Migration path
When talking about migration we refer to the way through which petroleum passes while it is migrating. Migration is the movement of newly formed hydrocarbon out of their source rock and this movement is generally known as primary migration. The movement into the hydrocarbon reservoir is known as secondary migration. Migration takes place from structurally low area to a higher area in the subsurface because of the relative buoyancy of hydrocarbons in comparison to the surrounding rock.
3. Reservoir rock
Reservoir as it is familiarly known is place where things are kept. Reservoir rock also is a rock on which the generated hydrocarbons are accumulated and stay there. Reservoir rock is characterized by high porosity and permeability these enable it to store and transmit hydrocarbons. Most common reservoir rocks are sedimentary rocks because they have more porosity compared to igneous and metamorphic rocks. Reservoir rocks form under condition on which hydrocarbons can be preserved. Reservoir rock is crucial element in a petroleum system.
4. Seal (Cap rock)
Cap rock is an impermeable rock that forms above or around a reservoir rock, and its primary role is to act as a barrier to further migration of hydrocarbons. A cap rock can be shale, anhydride, and salt.
Image showing source rock, reservoir rock, and cap rock5
Trap is a configuration of rocks suitable for containing hydrocarbons and sealed by a relatively impermeable formation through which hydrocarbons will not migrate. Traps are described as structural trap this type of traps are formed in geologic structure like fault and fold. Stratigraphic trap this kind of traps result from changes in rock type.
To the above mentioned elements appropriate timing and adequate temperature is also required to the generation of hydrocarbons, its accumulation and finally preservation.